Hybrid Seal Dual Runner

ABSTRACT

A turbomachine has a bearing in a bearing compartment. A first seal system isolates the bearing compartment and has: a first runner; a second runner; an inner seal encircling the first runner; and an outer seal encircling the second runner. The inner seal has a static clearance with the first runner. The outer seal has a static clearance with the second runner, greater than the inner seal static clearance. The first runner has an inner portion cantilevered inward. The second runner has an outer portion cantilevered outward.

U.S. GOVERNMENT RIGHTS

This invention was made with Government support. The Government hascertain rights in this invention.

BACKGROUND

The disclosure relates to gas turbine engines. More particularly, thedisclosure relates to seals for isolating oil-containing bearingcompartments.

Gas turbine engines (used in propulsion and power applications andbroadly inclusive of turbojets, turboprops, turbofans, turboshafts,industrial gas turbines, and the like) include multiple main bearings(e.g., rolling element bearings—thrust and/or radial) supporting onespool relative to another or relative to grounded structure (e.g. acase).

The bearings are exposed to oil for lubrication and/or cooling. The oilmay be passed as a recirculating flow that is passed to the bearings andthen collected (scavenged) and ultimately returned to the bearings. Toisolate the oil to bearing compartments, each associated with andcontaining one or more of the bearings, seal systems are used. Exampleseal systems are carbon seal systems.

To limit oil leakage past the seals, the seals may be buffered bydelivering air (e.g., bleed air) to spaces opposite the compartmentsacross the seals so that the compartments are at a lower pressure thanthe pressurized spaces.

In one group of two-spool engine configurations, a particularly relevantbearing compartment is located radially under the combustor, forward ofthe high pressure turbine (HPT). The high-pressure buffer air taken fromthe high pressure compressor (HPC) may pass through a cooler and haveits pressure stepped down in one or more stages before entering thebearing compartment. A target delta pressure difference relative to thecompartment may be maintained to limit any oil leakage. In one example,a two stage contacting carbon seal allows the required pressure drop.Notwithstanding the pressure difference, some oil will bypass the oilside seal and be evacuated (scuppered) from the interstage. Thescuppered oil may be disposed of to prevent fire within the engine. Themixture of buffer air and scuppered oil may be passed to a bypassflowpath or otherwise dumped or may be routed back into the gaspath.

Due to a high thermal gradient across the seal runner and moments causedby operation, there is an increased wear rate on the oil side sealresulting in premature replacement/removal of the seal.

U.S. Pat. No. 10,502,094, Shuaib et al., Dec. 10, 2019, “Bearingcompartment sealing system with passive cooling”, discloses two-stageseals of hybrid form in that the inner (oil-side) seal is a contactingseal and the outer (air-side) seal is a non-contacting seal.

SUMMARY

One aspect of the disclosure involves a turbomachine comprising abearing in a bearing compartment. A first seal system isolates thebearing compartment and has: a first runner; a second runner; an innerseal encircling the first runner; and an outer seal encircling thesecond runner. The inner seal has a static clearance with the firstrunner. The outer seal has a static clearance with the second runner,greater than the inner seal static clearance. The first runner comprisesan inner portion cantilevered inward. The second runner comprises anouter portion cantilevered outward.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the inner seal static clearance iszero.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the outer seal static clearance is0.0010 inch to 0.0050 inch (0.025 to 0.13 mm) radially.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the inner seal is a non-archboundseal and the outer seal is an archbound seal.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the inner seal a circumferentialspring biasing the inner seal into engagement with the first runner.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the inner seal is a carbon seal andthe outer seal is a carbon seal.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the inner seal is circumferentiallysegmented and the outer seal is circumferentially segmented.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the first runner comprises an innerportion cantilevered inward and the second runner comprises an outerportion cantilevered outward.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the first runner has a mountingflange and the second runner has a mounting flange abutting the innerportion mounting flange.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the first runner has a firstcoefficient of thermal expansion and the second runner has secondcoefficient of thermal expansion lower than the first coefficient ofthermal expansion.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the turbomachine further comprises anoil source positioned to direct oil to preferentially cool the innerportion relative to the outer portion.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the turbomachine further comprises abuffer air source introducing air past the outer seal to an interstage.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the turbomachine further comprises anoutlet from the interstage.

A further aspect of the disclosure involves a method for using theturbomachine, the method comprising: driving rotation of a shaftcarrying the first runner and the second runner; and passing an airflowbetween outer seal and the second runner to mix with an oil leakage flowin an interstage of the seal system.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the method further comprisesdirecting a cooling oil flow to an inner diameter surface of the firstrunner inner portion.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the first runner inner portionthermally deforms to cone inward in the inward direction and the secondrunner outer portion thermally deforms to cone outward in the outwarddirection.

A further aspect of the disclosure involves a turbomachine comprising abearing in a bearing compartment. A first seal system isolates thebearing compartment and has: a runner; an inner seal encircling therunner; and an outer seal encircling the runner. The runner has: a firstpiece encircled by the inner seal; and a second piece encircled by theouter seal.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the runner first piece comprises aninner portion cantilevered inward and the runner second piece comprisesan outer portion cantilevered outward.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively: an oil source is positioned to directan oil flow to an inner diameter surface of the runner first piece innerportion; and the turbomachine lacks an oil source positioned to directan oil flow to an inner diameter surface of the runner second pieceouter portion.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the runner first piece has a firstcoefficient of thermal expansion and the runner second piece has secondcoefficient of thermal expansion lower than the first coefficient ofthermal expansion.

In a further embodiment of any of the foregoing embodiments,additionally and/or alternatively, the runner first piece comprisesstainless steel and the runner second piece a molybdenum alloy or aceramic.

Another aspect of the disclosure involves a turbomachine comprising abearing in a bearing compartment. A first seal system isolates thebearing compartment and comprises: a first runner; a second runner; aninner seal encircling the first runner; and an outer seal encircling thesecond runner. The inner seal has a static clearance with the firstrunner. The outer seal has a static clearance with the second runner,greater than the inner seal static clearance. The turbomachine includesmeans for preferentially cooling the first runner.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial partially schematic central axial sectional view ofa seal system.

FIG. 1A is an enlarged view of the seal system of FIG. 1 showingoperational deflections in broken lines.

FIG. 2 is a view of a seal segment of the seal system.

FIG. 3 is a schematic view of a buffering and scuppering system.

FIG. 4 is a schematic view of a gas turbine engine in which the sealsystem may be included.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a seal system 100 having a first member 102 carrying a pairof seals 104, 105. As is discussed further below, the seal system isused in a turbomachine such as a gas turbine engine for a purpose suchas isolating a bearing compartment. The example seals 104, 105 arecarbon seals having an inner diameter (ID) seal surface or face 106,107. The example seals 104, 105 are each formed as single-piece orsegmented body of revolution about an axis 500 which is an axis ofrelative rotation between the first member 102 and a second member 110.As is discussed further below, in an example implementation, the firstmember 102 is engine static structure and the second member 110 is ashaft assembly (e.g., of a high spool shaft 111).

FIG. 1 further shows an outward radial direction 502 and a forwarddirection 504. The second member 110 comprises one or more pieces 112,113 (runner pieces or just runners) forming runners for the seals 104,105. The runner pieces have respective outer diameter (OD) surfaces orfaces 114, 115 radially facing the respective seal ID surfaces 106, 107.The seals and runners have respective radial clearances labeled R_(CI)and R_(CO).

The seal system 100 isolates a space or volume 600. The example space orvolume 600 is a bearing compartment. The example seal system is at aforward end of the bearing compartment. A similar seal system may be atan aft end. The example bearing compartment 600 contains a bearingsupporting the second member 110 for rotation relative to the firstmember 102 about the axis 500. Relative to the bearing compartment 600,the seal 104 is an inner or inboard seal and the seal 105 is an outer oroutboard seal.

The example seal system 100 immediately isolates the bearing compartment600 from a second space or volume 602. The example second space orvolume 602 is a buffer chamber which, itself, is separated from a spaceor volume 604 forward thereof via a seal system 120 (e.g., a knife edgeseal system). The example seal system 120 comprises annular knife edgeson one end section of a spacer 122 interfacing with an axial sleevesection of a heat shield 124.

As is discussed further below, the bearing compartment 600 contains oilwhich serves various lubrication and/or cooling functions includingbearing lubrication and/or cooling and seal lubrication and/or cooling.Thus, the seal 104 and runner 112 are an oil side seal and runner andthe seal 105 and runner 113 are an air side seal and runner. FIG. 1shows a leakage flow 400 exiting the bearing compartment 600. To limitthe leakage flow and direct it for collection, a buffer air flow 402 isintroduced passing through the buffer chamber 602. A portion 402A of thebuffer air flow may pass into the bearing compartment 600 due to thepressure difference between bearing compartment and interstage (plenum606 discussed below) of the seal system 100. The leakage flow merges inthe interstage with a remainder 402B of the buffer air flow to form acombined outlet flow or flows 406. The flow(s) 406 may be exhausted toatmosphere and/or combusted. The composition of the flow 406 may vary inoperation. For example, there may only be oil in the flow 406 duringcertain transient conditions of operation, such as engine startup orshutdown, or perhaps if the oil side seal wears and nears the end oflife.

In addition to oil delivered to the bearing(s), the flow 400 may furthercome from oil 410 used for seal cooling and/or lubrication. FIG. 1 showsa nozzle 126 having an outlet 128 spraying oil 410 for cooling the oilside runner 112. There may be a circumferentially-distributed pluralityof such nozzles.

Each example runner piece 112, 113 comprises a mounting flange 130, 131and an axially cantilevered portion 132, 133. The cantilevered portionsare cantilevered from the associated flange viewed in section so as toextend to free ends. The cantilevered portion 132 is cantilevered inwardinto the bearing compartment 600; whereas, the cantilevered portion 133is cantilevered outward therefrom. The respective flanges have an innerdiameter (ID) surface 134, 135. The cantilevered portions extend fromjunctions with the flanges to distal end surfaces 136, 137 formingrespective end rims. The cantilevered portions have respective innerdiameter surfaces 138, 139. The flanges have first faces 140, 141contacting and/or otherwise facing each other (e.g., potentiallyseparated via gasketing, a spacer, or the like). The flanges furtherhave opposite faces 142, 143. The flanges 130,131 are sandwiched betweenend sections of supports 144, 122 for axial retention and have innerdiameter (ID) surfaces in interference engagement with the high spoolshaft 111 for rotational capture. The supports, themselves are capturedby sections of the high spool shaft 111.

As distinguished from a shared runner with a single compartment to theID of both seals, the flange structure with opposed cantilever portionsisolates cooling of the inner runner 112 from cooling, if any, of theouter runner 113. This allows the oil spray/flow 410 to preferentiallycool the inner runner relative to the outer runner.

Exemplary runner materials are alloys (e.g., stainless steel). The tworunners may be of different alloys or different materials. For example,the oil side runner may comprise stainless steel and the air side runnermay comprise a lower coefficient of thermal expansion (CTE) materialsuch as a molybdenum alloy or a ceramic. An example ceramic is siliconnitride with a CTE about 2×10−6/° F. versus about 9×10−6/° F. for manystainless steels. An example CTE of the lower CTE material is 15 to 90percent that of the higher CTE material, more narrowly 15 to 50 percentfor non-metallic air side runners and metallic oil side or 25 to 90percent if both are metallic.

The carbon seals further comprise outer diameter (OD) surfaces 160, 161,first radial faces 162, 163, and second radial faces 164, 165. The ODsurfaces include annular grooves 166, 167 accommodating respectivesprings and/or seals 170, 171. As is discussed further below, theexample springs 170, 171 are full annulus coil springs in tension tohold segments of the seals together. Depending upon implementation, thesprings may be unjacketed or may have an elastic tubular jacket thatalso provides sealing.

FIG. 2 shows one example of segment ends for forming junctions betweensegments. The mating ends of seal segments may have various interfittingfeatures such as tongue/groove or shiplap features that still providesome sealing over a range of relative segment position. An examplenumber of segments per seal is three to twenty, more particularly, fourto eight. The segments have first and second circumferential ends 190191. The example segment ends have a generally radial/axial main face192, 193. One end has a tongue (projection) 194 and the other end has agroove (compartment) 195 for receiving the tongue of the adjacentsegment end. The exemplary tongue and groove of generally rectangularsection at one corner of the generally rectangular section of the end.

The carbon seals 104, 105 are accommodated in respective compartments172, 173 (FIG. 1A) in the first member. The example first membercomprises multiple pieces. The respective fore-and-aft faces of thecarbon seals are captured by associated faces of portions of the firstmember and the respective OD surfaces 160, 161 are captured byrespective ID surface portions 180, 181.

The example seal 105 is an archbound (AB) seal wherein with its segmentsabutting end-to-end there is still available radial clearance with theassociated runner 113. Example centered static R_(CO) may be at leastabout 0.0010 inch (0.025 mm) for gas turbine engine use (e.g., 0.0010 to0.010 inch (0.025 to 0.25 mm) or 0.0010 to 0.0050 inch (0.025 to 0.13mm) or 0.0020 inch to 0.0050 inch (0.051 to 0.13 mm)). This may be inexcess of ten times R_(CI) if R_(CI) is non zero. There may,additionally, be OD radial clearance with the first member or there maybe a press/interference fit relationship with the first member. Theexample seal 104 is non-archbound so that the ID surface 106 along thevarious segments may simultaneously contact the associated runner 112.Thus, the inner seal 104 may have OD radial clearance with the firstmember.

A first piece of the example first member is a seal support such asformed by a hub structure 200 extending radially outward to the gas path(e.g., to struts or nozzles downstream of the combustor and upstream ofthe high pressure turbine (HPT)). Vanes or struts may pass loads fromthe hub through to outer static case structure.

A second piece 202 of the first member is a seal housing for the outerseal 105 and acts as an axial retainer for the inner seal 104. Theexample hub 200 has surfaces 180, 210 forming the OD and one radial face(axial end face) of the compartment 172. The surface 210 is along aninwardly-directed flange 212. The opposite face of the compartment 172is formed by an end surface 214 of a flange 215 of the seal housing 202.A radially inboard portion of the surface 214 bounds the compartment 172and a radially outboard portion abuts a shoulder of the hub.

The seal housing 202 further includes an outer sleeve section 220 havingan OD surface 222 accommodated in a compartment of the hub having an IDsurface 224. A portion of an ID surface of the sleeve section 220 formsthe surface 181. The seal housing further includes a second flange 230having a surface 232 forming one end of the compartment 173. A secondend is formed by a retainer clip 234 having an OD portion captured in anID groove in the seal housing. Similarly, a second clip 236 captures theseal housing 202. An annular channel between the flanges 215 and 230forms an interstage collection plenum 606 with one or more meteringorifices 240 at a radially outward base of the channel forming theplenum. The orifices meter and pass the flow 406 to a passageway 242 inthe hub for ultimate disposal as discussed above.

FIG. 1A shows an in-service/operational condition of the runners andseals in broken lines. Relative to the solid line static condition, theoil side runner is shown having a thermally expanded flange.

Due to cooling from the oil spray 410 from one or more cooling outlets128, the cantilevered portion 132 thermally contracts to counter theeffect of flange thermal expansion. The result is that the cantileveredportion 132 cones inward. The use of two runners helps concentrate theoil cooling along the cantilevered portion adjacent the oil-side sealrelative to an alternative longer cantilevered portion spanning bothseals. In distinction, the air side runner is shown with less flangethermal expansion. But, due to the lack of active cooling, there may bean outward coning of the cantilevered portion 133.

For the oil side runner, the resultant coned state of the runner willreduce the pressure loading on the oil side seal. Reducing the pressureloading on the oil side will increase the durability of the oil sideseal. From a pressure loading standpoint, the contacting carbon willexperience a lower pressure if the runner cones divergently from theseal in the direction of buffer air flow (so that the surface 114converges from proximal to distal and inward into the bearingcompartment) because segments will conform to the runner duringoperation.

For the air side seal runner, a separate runner may reduce sensitivityto the varying ranges of speed and pressure conditions throughout theoperating envelope. The thermals of the runner are directly tied to thespeed and pressure conditions. Having a lower sensitivity to theseparameters such as via a lower CTE material allows for a smaller rangeof radial gap variation. This does not need to be complemented with aseparate cooling oil delivery system. This allows for better tuning ofthe system level impacts due to the resultant air leakage past the seal.In various implementations, compared with a baseline seal system one orboth of the configuration and difference in runner materials may reduceoverall runner deflections experienced by the oil-side seal whilemaintaining the required pressure difference.

FIG. 3 shows a buffering and scuppering system 300 serving the bearingcompartment 600 containing a bearing 302 (e.g., a rolling elementbearing). Seal system 100 is to one side of the bearing compartment anda seal system 100′ is opposite. The seal system 100′ may be similar tothe seal system 100 having similar seals 104′, 105′ 120′, similar spaces602′, 604′, 606′, and similar flows and flowpaths/lines. A buffer airsource 310 may be an HPC bleed as discussed above. An oil/air scupperingdestination 312 may be an air/gas stream such as a return to the coreflowpath at a lower pressure stage in the turbine (e.g., the lowpressure turbine (LPT) or a lower pressure stage within the highpressure turbine (HPT). FIG. 3 further shows a recirculating oil supplysystem 320 which may include pumps, reservoirs, filters, cooling heatexchangers, and the like. The oil supply system 320 scavenges andrecirculates oil for bearing lubrication/cooling and seal runnercooling.

Component manufacture and assembly techniques may be otherwiseconventional. Metallic runners may be machined (e.g., on a lathe orturning machine from stock material or from a rough casting. Metallicrunners may be assembled to the shaft via thermal interference fit andor inter-fitting anti-rotation features such as splines (not shown).Non-metallic runners (e.g., ceramic or ceramic matrix composite (CMC)may be molded and finish machined (e.g., on a lathe or turning machine).Such non-metallic runners may have anti-rotation features mated tocorresponding shaft features or corresponding features of the metallicrunner or spacer (e.g., castellation features—not shown) to limitpotential cracking from thermal interference.

FIG. 4 schematically illustrates a gas turbine engine 20 as one of manyexamples of an engine in which the seal system 100 may be used. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. The fan section 22 mayinclude a single-stage fan 42 having a plurality of fan blades 43. Thefan blades 43 may have a fixed stagger angle or may have a variablepitch to direct incoming airflow from an engine inlet. The fan 42 drivesair along a bypass flow path B in a bypass duct 13 defined within ahousing 15 such as a fan case or nacelle, and also drives air along acore flow path C for compression and communication into the combustorsection 26 then expansion through the turbine section 28. A splitter 29aft of the fan 42 divides the air between the bypass flow path B and thecore flow path C. The housing 15 may surround the fan 42 to establish anouter diameter of the bypass duct 13. The splitter 29 may establish aninner diameter of the bypass duct 13. Although depicted as a two-spoolturbofan gas turbine engine in the disclosed non-limiting embodiment, itshould be understood that the concepts described herein are not limitedto use with two-spool turbofans as the teachings may be applied to othertypes of turbine engines including three-spool architectures.

The example engine 20 generally includes a low speed spool 30 and a highspeed spool 32 mounted for rotation about an engine central longitudinalaxis A relative to an engine static structure 36 via several bearingsystems 38. It should be understood that various bearing systems 38 atvarious locations may alternatively or additionally be provided, and thelocation of bearing systems 38 may be varied as appropriate to theapplication.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects, a first (or low) pressure compressor 44 and a first (orlow) pressure turbine 46. The inner shaft 40 is connected to the fan 42through a speed change mechanism, which in the example gas turbineengine 20 is illustrated as a geared architecture 48 to drive the fan 42at a lower speed than the low speed spool 30. The inner shaft 40 mayinterconnect the low pressure compressor (LPC) 44 and low pressureturbine (LPT) 46 such that the low pressure compressor 44 and lowpressure turbine 46 are rotatable at a common speed and in a commondirection. In other embodiments, the low pressure turbine 46 drives boththe fan 42 and low pressure compressor 44 through the gearedarchitecture 48 such that the fan 42 and low pressure compressor 44 arerotatable at a common speed. Although this application discloses gearedarchitecture 48, its teaching may benefit direct drive engines having nogeared architecture. The high speed spool 32 includes an outer shaft 50that interconnects a second (or high) pressure compressor (HPC) 52 and asecond (or high) pressure turbine (HPT) 54. A combustor 56 is arrangedin the example gas turbine 20 between the high pressure compressor 52and the high pressure turbine 54. A mid-turbine frame 57 of the enginestatic structure 36 may be arranged generally between the high pressureturbine 54 and the low pressure turbine 46. The mid-turbine frame 57further supports bearing systems 38 in the turbine section 28. The innershaft 40 and the outer shaft 50 are concentric and rotate via bearingsystems 38 about the engine central longitudinal axis A which iscollinear with their longitudinal axes.

Airflow in the core flow path C is compressed by the low pressurecompressor 44 then the high pressure compressor 52, mixed and burnedwith fuel in the combustor 56, then expanded through the high pressureturbine 54 and low pressure turbine 46. The mid-turbine frame 57includes airfoils 59 which are in the core flow path C. The turbines 46,54 rotationally drive the respective low speed spool 30 and high speedspool 32 in response to the expansion. It will be appreciated that eachof the positions of the fan section 22, compressor section 24, combustorsection 26, turbine section 28, and fan drive gear system 48 may bevaried. For example, gear system 48 may be located aft of the lowpressure compressor, or aft of the combustor section 26 or even aft ofturbine section 28, and fan 42 may be positioned forward or aft of thelocation of gear system 48.

The low pressure compressor 44, high pressure compressor 52, highpressure turbine 54 and low pressure turbine 46 each include one or morestages having a row of rotatable airfoils. Each stage may include a rowof static vanes adjacent the rotatable airfoils. The rotatable airfoilsand vanes are schematically indicated at 47 and 49.

The use of “first”, “second”, and the like in the following claims isfor differentiation within the claim only and does not necessarilyindicate relative or absolute importance or temporal order. Similarly,the identification in a claim of one element as “first” (or the like)does not preclude such “first” element from identifying an element thatis referred to as “second” (or the like) in another claim or in thedescription.

Where a measure is given in English units followed by a parentheticalcontaining SI or other units, the parenthetical's units are a conversionand should not imply a degree of precision not found in the Englishunits.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, whenapplied to an existing or new baseline engine or seal configuration,details of such baseline may influence details of particularimplementations. Accordingly, other embodiments are within the scope ofthe following claims.

1. A turbomachine comprising: a bearing in a bearing compartment; and afirst seal system isolating the bearing compartment and comprising: afirst runner; a second runner; an inner seal encircling the firstrunner; and an outer seal encircling the second runner, wherein: theinner seal has a static clearance with the first runner; the outer sealhas a static clearance with the second runner, greater than the innerseal static clearance; the first runner comprises an inner portioncantilevered inward; the second runner comprises an outer portioncantilevered outward, and the turbomachine further comprises an oilsource positioned to direct oil to preferentially cool the inner portionrelative to the outer portion.
 2. The turbomachine of claim 1 wherein:the inner seal static clearance is zero.
 3. The turbomachine of claim 2wherein: the outer seal static clearance is 0.0010 inch to 0.0050 inch(0.025 to 0.13 mm) radially.
 4. The turbomachine of claim 1 wherein: theinner seal is a non-archbound seal; and the outer seal is an archboundseal.
 5. The turbomachine of claim 1 further comprising: acircumferential spring biasing the inner seal into engagement with thefirst runner.
 6. The turbomachine of claim 1 wherein: the inner seal isa carbon seal; and the outer seal is a carbon seal.
 7. The turbomachineof claim 1 wherein: the inner seal is circumferentially segmented; andthe outer seal is circumferentially segmented.
 8. The turbomachine ofclaim 1 wherein: the first runner comprises an inner portioncantilevered inward; and the second runner comprises an outer portioncantilevered outward.
 9. The turbomachine of claim 1 wherein: the firstrunner has a mounting flange; and the second runner has a mountingflange abutting the inner portion mounting flange.
 10. The turbomachineof claim 1 wherein: the first runner has a first coefficient of thermalexpansion; and the second runner has second coefficient of thermalexpansion lower than the first coefficient of thermal expansion. 11.(canceled)
 12. The turbomachine of claim 1 further comprising: a bufferair source introducing air past the outer seal to an interstage; and anoutlet from the interstage.
 13. (canceled)
 14. A method for using theturbomachine of claim 1, the method comprising: driving rotation of ashaft carrying the first runner and the second runner; and passing anairflow between the outer seal and the second runner to mix with an oilleakage flow in an interstage of the seal system.
 15. The method ofclaim 14 further comprising: directing a cooling oil flow to an innerdiameter surface of the first runner inner portion.
 16. The method ofclaim 14 wherein: the first runner inner portion thermally deforms tocone inward in the inward direction; and the second runner outer portionthermally deforms to cone outward in the outward direction.
 17. Aturbomachine comprising: a bearing in a bearing compartment; and a firstseal system isolating the bearing compartment and comprising: a runner;an inner seal encircling the runner; and an outer seal encircling therunner, wherein the runner has: a first piece encircled by the innerseal; and a second piece encircled by the outer seal.
 18. Theturbomachine of claim 17 wherein: the runner first piece comprises aninner portion cantilevered inward; and the runner second piece comprisesan outer portion cantilevered outward.
 19. The turbomachine of claim 18wherein: an oil source is positioned to direct an oil flow to an innerdiameter surface of the runner first piece inner portion; and theturbomachine lacks an oil source positioned to direct an oil flow to aninner diameter surface of the runner second piece outer portion.
 20. Theturbomachine of claim 17 wherein: the runner first piece has a firstcoefficient of thermal expansion; and the runner second piece has secondcoefficient of thermal expansion lower than the first coefficient ofthermal expansion.
 21. The turbomachine of claim 17 wherein: the runnerfirst piece comprises stainless steel; and the runner second piece amolybdenum alloy or a ceramic.
 22. A turbomachine comprising: a bearingin a bearing compartment; and a first seal system isolating the bearingcompartment and comprising: a first runner; a second runner; an innerseal encircling the first runner; and an outer seal encircling thesecond runner, wherein: the inner seal has a static clearance with thefirst runner; the outer seal has a static clearance with the secondrunner, greater than the inner seal static clearance; the turbomachineincludes means for preferentially cooling the first runner.